I don't think so: there are several wallets with the same issue, not just Sia. I would say that it's due to increasing demand on the Polo platform.

On the other hand, the recent rise of the bitcoin was after the 700: the spike in the price of btc put almost all altcoins in red, dropping Sia's under 500. When bitcoin's price grow slowly altcoins grow with it but when there are big rushes in bitcoin they don't.